1. Field of the Invention
The present invention relates to an index position detecting apparatus for an electric rotary machine such as a motor used in a floppy disk drive or the like, and more particularly to improvements in detection of the index position of the rotary machine and of timing for generating a rotating magnetic field.
2. Description of the Prior Art
As a conventional apparatus for detecting the rotating position of a motor, one that employs Hall effect sensors is known. The Hall effect sensors are disposed at positions facing magnetic poles of the driving magnet of a rotor so as to detect changes in the magnetic flux of the magnet, and the output signals of the Hall effect sensors are used as timing signals for producing the rotating magnetic field of the motor.
Likewise, as a conventional apparatus for detecting the index position of a motor, one that employs magnetic detector such as a Hall effect sensor is known. The Hall effect sensor is disposed on a stator of the motor so as to encounter a magnet fixed on the rotor of the motor. The sensor detects changes in the magnetic flux of the magnet and produces a pulse signal which is used as a rotating position signal indicating the index position.
Referring to drawings, a conventional motor will be described. FIG. 1 is a cross-sectional plan view showing the major portion of a conventional three-phase brushless motor, and FIG. 2 is a cross-sectional view taken along line X--X of FIG. 1. The conventional brushless motor has four magnetic circuits: a magnetic circuit for generating a rotating driving force; that for generating a frequency generating signal; that for generating an index signal indicating the rotating position of the rotor; and that for detecting exciting timings for generating the rotating magnetic field.
The construction of the three-phase brushless motor is shown schematically in FIG. 2. An oilless bearing 9 is press-inserted at the center of a substrate 7 made of a magnetic material such as iron, and a Hall effect sensor 14 functioning as an index position detecting device is disposed at the periphery of the substrate 7. A rotating shaft 5 on which a rotor yoke 4 is mounted via a shaft fixing member 6 is fixed into the inner ring of a bearing 8 and the oilless bearing 9. Thus, an integral structure including the rotor yoke 4, a driving magnet 1, a frequency generating magnet 2, and an index magnet 13 can freely rotate with respect to the substrate 7.
The driving magnet 1 is fixed on the inner wall of the periphery of the rotor yoke 4. As is well known, the rotor yoke 4 is driven to rotate by applying a rotating magnetic field on the driving magnet 1. The driving magnet 1 is subjected to a 16-pole magnetization along the circumference thereof, and is fixed on the inner wall of the periphery of the rotor yoke 4.
To produce the rotary magnetic field, a plurality of driving coils 10 are wound on stator yokes 11 which are radially formed around the rotating shaft 5. The stator yokes are fixed on the substrate 7 with fixing members such as screws.
In this structure, the stator yokes 11 form a close magnetic circuit together with the driving magnet 1, a driving magnet yoke 3, and the rotor yoke 4. Here, this type of brushless motor, wherein the driving magnet 1 is disposed in opposition to the circumference of the stator yokes 11 making a small gap between them, is called a circumference-opposing type motor.
In the outer wall of the periphery of the rotor yoke 4, a notch 4h is formed into which the index magnet 13 is embedded functioning as a part of a rotation position detecting means. The index magnet 13 forms another magnetic circuit.
Furthermore, a plurality of Hall effect sensors 12a, 12b and 12c are fixed at appropriate positions on the substrate 7 in order to detect the exciting timings of coils 10 of respective phases. The Hall effect sensors 12a, 12b and 12c detect changes in the magnetic flux caused by the passage of the driving magnet 1. Thus, the phase difference between the magnetic field produced by the coils 10 and the magnetic field produced by the rotating driving magnet 1 is detected so that each driving coil 10 is supplied with a current at appropriate timings to produce the rotating magnetic field. This rotating field rotates the rotor 4 in the direction shown by the arrow A in FIG. 1.
The frequency generating magnet 2, on the other hand, has a total of 120 magnetized poles, and is attached to the outer wall of the periphery of the rotor yoke 4. Opposing the frequency generating magnet 1, there is provided a generating wire 7a on the surface of the substrate 7. The generating wire a consists of 120 U-shaped elements corresponding to the number of poles of the frequency generating magnet 2, as shown in FIG. 3. The generating wire 7a is made of copper or the like etched on the substrate 7.
With this structure, when the rotor yoke 4 is driven to rotate, the generating wire 7a generates a sine wave of a frequency corresponding to the rotating speed of the rotor yoke 4. Thus, a control circuit not shown carries out a constant speed control.
When the rotor yoke 4 rotates, the index magnet 13 fixed in the rotor yoke 4 rotates with it so that the Hall effect sensor 14 for detecting index position senses the flux change of the magnet 13, and produces an index signal consisting of one pulse for each rotation of the rotor yoke 4, thus making it possible to detect the rotating phase of the rotor.
Furthermore, when the rotor yoke 4 is driven, the Hall effect sensors 12a, 12b and 12c for detecting the exciting timings of respective phases produce waveforms as shown in FIG. 4. On the basis of these signals, a coil driving signal synthesizing circuit 35 creates timing signals for exciting the driving coils 10U, 10V and 10W, and energizes coil driving amplifiers 36a, 36b and 36c. Thus, currents as shown in FIG. 6 flow through the driving coils 10U, 10V and 10W.
The Hall effect sensor 14, which detects the index position and generates the index signal, produces a waveform as shown in FIG. 7. This waveform is inputted to a comparator 15 as shown in FIG. 8 so that a waveform shown in the bottom of FIG. 7 is produced at the output of the comparator 15 as the index signal.
The conventional method of detecting the index position described above has the following problems:
(1) Since the index magnet 13 is attached to the outer wall of the periphery of the rotor yoke 4, and the magnetic circuit formed by the index magnet 13 is open, the magnetic flux originating from the index magnet 13 works as leakage flux. As a result, when the motor is employed to rotate a disk in a magnetic recording and reproducing apparatus, correct recording and reproduction of information may be degraded by the leakage flux entering a magnetic head for recording and reproduction. PA0 (2) Spaces for providing the index magnet 13 and the Hall effect sensor 14 for detecting the index position are necessary. This prevents the multiple phase brushless motor from being made smaller in size and thinner in thickness, and in addition, makes the cost higher.
Furthermore, the above-described conventional method of detecting timings for driving the coils 10 necessitates spaces for providing the Hall effect sensors 12a, 12b and 12c for detecting the timings. This also prevents the multiple phase brushless motor from being made smaller and thinner. In addition, this increases the cost of the motor.